This application is based on Japanese patent application No. 2000-150185, the contents of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display which carries out matrix driving of a chiral nematic liquid crystal composition, which is capable of displaying an image thereon continuously after a turn-off of the electric field thereto, through a plurality of scan electrodes and a plurality of data electrodes which face and cross each other.
2. Description of Related Art
In recent years, reflective type liquid crystal displays which use a chiral nematic liquid crystal composition which is produced by adding a chiral agent to nematic liquid crystal and which exhibits a cholesteric phase at room temperature are developed to be used as media for reproducing digital information into visual information. Such liquid crystal displays have a memory effect and accordingly consume little electric power. Also, such liquid crystal displays can be fabricated at low cost.
Although the liquid crystal displays of this kind have these advantages, they have a demerit that the driving speed is low. In order to solve this problem, there have been suggested some methods of driving such a liquid crystal display at a high speed. For example, U.S. Pat. No. 5,748,277 disclosed a method which comprises a preparation phase to cause the liquid crystal to come to a homeotropic state, a selection phase to select the liquid crystal to come to a focal-conic state or a planar state and an evolution phase to cause the liquid crystal to evolve to the desired final state.
However, liquid crystal materials which are suited to be subjected to such a driving method are yet to be studied sufficiently, and liquid crystal compositions which are satisfactory in reflectance, in contrast between a planar state and a focal-conic state, in color purity (excitation purity), etc. have not been specified yet.
An object of the present invention is to provide a reflective type liquid crystal display which is good in bistability, in color purity, in reflectance, etc., thereby resulting in display of an image with high contrast, which has a wide temperature compensation range and which can be driven at a high speed.
In order to attain the object, the inventors studied about the composition of chiral nematic liquid crystal. As a result, the inventors found out that when a chiral nematic liquid crystal composition is to be driven by a method which comprises a reset step of resetting the chiral nematic liquid crystal composition to a homeotropic state, a selection step of selecting the final state of the chiral nematic liquid crystal composition and an evolution step of causing the chiral nematic liquid crystal composition to evolve to the selected final state, the anisotropy of dielectric constant, the anisotropy of refractive index and the viscosity of the chiral nematic liquid crystal composition must be balanced with one another. Accordingly, a liquid crystal display according to the present invention uses nematic liquid crystal which contains a liquid crystalline ester compound expressed by the following formula (A) or (Axe2x80x2), a liquid crystalline stilbene compound expressed by the following formula (B), a liquid crystalline tarphenyl compound expressed by the following formula (C) or a liquid crystalline tolane compound expressed by the following formula (D). 
R2: alkyl group, alkoxyl group, alkenyl group or fluoroalkyl group
Q: alkyl group, alkoxyl group, alkenyl group, cyano group, fluorine atom or fluoroalkyl group
J, K: 1, 4-phenylene group or 1, 4-cyclohexylene group
m, n: integer from 0 to 4
R2: alkyl group, alkoxyl group, alkenyl group or fluoroalkyl group
Q: alkyl group, alkoxyl group, alkenyl group, cyano group, fluorine atom or fluoroalkyl group
J, K, P: 1, 4-phenylene group or 1, 4-cyclohexylene group
l, m, n: integer from 0 to 4
R1: alkyl group with 1 to 10 carbons or alkenyl group with 2 to 10 carbons
A: single bond or 1, 4-cyclohexylene group
X: xe2x80x94F, xe2x80x94C1 or xe2x80x94CN
Y, Z: xe2x80x94F or xe2x80x94H 
R3: alkyl group, alkoxyl group or alkenyl group
T: cyano group, halogen atom, alkyl group or alkoxyl group
v, u: integer from 0 to 4
R4: alkyl group or alkoxyl group
W: alkyl group, alkoxyl group, fluorine atom fluoroalkyl group or fluoroalkoxyl group
B: 1, 4-phenylene group, 1, 4-cyclohexylene group or single bond
E: 1, 4-cyclohexylene group or single bond
Liquid crystalline ester compounds and liquid crystalline stilbene compounds are high in anisotropy of dielectric constant. If nematic liquid crystal contains such a compound, the liquid crystal improves in responsibility to a voltage applied thereto, and consequently, it becomes possible to lower the driving voltage. Also, this compound improves the performance in displaying black, and consequently, the contrast becomes higher. Liquid crystalline terphenyl compounds are high in anisotropy of refractive index. If nematic liquid crystal contains such a compound, the liquid crystal consequently includes lots of scattering elements, and the contrast becomes higher. Also, because of this compound, the operating temperature range of the liquid crystal becomes wider. Further, because of this compound, the liquid crystal is capable of maintaining reliability and stability for a long time. Liquid crystalline tolane compounds are high in birefringence. If nematic liquid crystal contains such a compound, the viscosity of the liquid crystal is lowered while the high birefringence is maintained. Further, by containing a liquid crystalline compound other than these kinds in nematic liquid crystal, which has a polar group at an end, the driving voltage can be lowered more.
Thus, these compounds permit liquid crystal to obtain good characteristics, such as high responsibility to the voltage applied, high contrast and low viscosity while keeping high birefringence. Therefore, when the liquid crystal is driven at a high speed by the driving method which comprises the reset step, the selection step and the evolution step, the liquid crystal achieves good display performance.
Therefore, the liquid crystal display according to the present invention comprises a driving section for driving such a liquid crystal composition by a driving method which comprises a reset step of applying a reset pulse to cause the liquid crystal composition to a homeotropic state, a selection step of applying a selection pulse to select the final state of the liquid crystal composition and an evolution step of applying an evolution pulse to cause the liquid crystal composition to evolve to the selected final state and in which the selection of the final state of the liquid crystal composition is carried out by modulating the selection pulse.
Because the liquid crystal composition is driven step by step in the reset step, in the selection step and in the evolution step, a desired image can be written on the liquid crystal composition at a relatively high speed. Moreover, by modulating the selection pulse, display of intermediate tones becomes possible.
The liquid crystal composition may further contain a coloring agent or a ultraviolet-ray absorber. The coloring agent adjusts the color purity of the liquid crystal composition. The ultraviolet-ray absorber prevents the liquid crystal composition from degrading because of being exposed to ultraviolet rays.
The driving method executed by the driving section may comprise, in the selection step, before and after the application of the selection pulse, a time of applying substantially zero volt to the liquid crystal composition. In this case, because there is a time of applying substantially zero volt in the selection step, the number of output levels of the driver can be reduced.